Mountain-Wave Turbulence in the Presence of Directional Wind Shear over the Rocky Mountains

Mountain-Wave Turbulence in the Presence of Directional Wind Shear over the Rocky Mountains
Guarino, Maria-Vittoria; Teixeira, Miguel A. C.; Keller, Teddie L.; Sharman, Robert D.
2018-04-26 00:00:00
AbstractMountain-wave turbulence in the presence of directional wind shear over the Rocky Mountains in Colorado is investigated. Pilot reports (PIREPs) are used to select cases in which moderate or severe turbulence encounters were reported in combination with significant directional wind shear in the upstream sounding from Grand Junction, Colorado (GJT). For a selected case, semi-idealized numerical simulations are carried out using the WRF-ARW atmospheric model, initialized with the GJT atmospheric sounding and a realistic but truncated orography profile. To isolate the role of directional wind shear in causing wave breaking, sensitivity tests are performed to exclude the variation of the atmospheric stability with height, the speed shear, and the mountain amplitude as dominant wave breaking mechanisms. Significant downwind transport of instabilities is detected in horizontal flow cross sections, resulting in mountain-wave-induced turbulence occurring at large horizontal distances from the first wave breaking point (and from the orography that generates the waves). The existence of an asymptotic wake, as predicted by Shutts for directional shear flows, is hypothesized to be responsible for this downwind transport. Critical levels induced by directional wind shear are further studied by taking 2D power spectra of the magnitude of the horizontal velocity perturbation field. In these spectra, a rotation of the most energetic wave modes with the background wind, as well as perpendicularity between the background wind vector and the wavenumber vector of those modes at critical levels, can be found, which is consistent with the mechanism expected to lead to wave breaking in directional shear flows.
http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.pngJournal of the Atmospheric SciencesAmerican Meteorological Societyhttp://www.deepdyve.com/lp/american-meteorological-society/mountain-wave-turbulence-in-the-presence-of-directional-wind-shear-Ug6fqMhz9I

Mountain-Wave Turbulence in the Presence of Directional Wind Shear over the Rocky Mountains

Abstract

AbstractMountain-wave turbulence in the presence of directional wind shear over the Rocky Mountains in Colorado is investigated. Pilot reports (PIREPs) are used to select cases in which moderate or severe turbulence encounters were reported in combination with significant directional wind shear in the upstream sounding from Grand Junction, Colorado (GJT). For a selected case, semi-idealized numerical simulations are carried out using the WRF-ARW atmospheric model, initialized with the GJT atmospheric sounding and a realistic but truncated orography profile. To isolate the role of directional wind shear in causing wave breaking, sensitivity tests are performed to exclude the variation of the atmospheric stability with height, the speed shear, and the mountain amplitude as dominant wave breaking mechanisms. Significant downwind transport of instabilities is detected in horizontal flow cross sections, resulting in mountain-wave-induced turbulence occurring at large horizontal distances from the first wave breaking point (and from the orography that generates the waves). The existence of an asymptotic wake, as predicted by Shutts for directional shear flows, is hypothesized to be responsible for this downwind transport. Critical levels induced by directional wind shear are further studied by taking 2D power spectra of the magnitude of the horizontal velocity perturbation field. In these spectra, a rotation of the most energetic wave modes with the background wind, as well as perpendicularity between the background wind vector and the wavenumber vector of those modes at critical levels, can be found, which is consistent with the mechanism expected to lead to wave breaking in directional shear flows.

Journal

Journal of the Atmospheric Sciences
– American Meteorological Society

Published: Apr 26, 2018

Recommended Articles

Loading...

References

Simulations of observed lee waves and rotor turbulence

Ágústsson, H.; Ólafsson, H.

The critical layer for internal gravity waves in a shear flow

Booker, J. R.; Bretherton, F. P.

Linear theory of momentum fluxes in 3-d flows with turning of the mean wind with height